Numerical Modelling of Front Contact Alignment for High Efficiency Cd1-xZnxTe and Cd1-xMgxTe Solar Cells for Tandem Devices

Abstract

AbstractWide bandgap Cd1-xZnxTe (CZT) and Cd1-xMgxTe (CMT) have drawn attention as top cells in tandem devices. These materials allow tuning of the band gap over a wide range by controlling the Zn or Mg concentration with little alteration to the base CdTe properties. Historically, CdS has been used as a heterojunction partner for CZT or CMT devices. However, these devices show a significant lower open circuit voltage (VOC) than expected for wide bandgap absorbers. Recent modelling work suggests that poor band alignment between the CdS emitter and absorber results in a high concentration of holes at the interface, which increased recombination and limits the VOC. This recombination should be exacerbated for wider bandgap absorbers such as CZT and CMT. In this study, we use numerical simulations with SCAPS-1D software to investigate the band alignment in the front contacts for wider bandgap CdTe based absorbers. Results show that by replacing the CdS with a wide bandgap emitter layer, the VOC can be greatly improved, though under certain conditions, the fill factor remains sensitive to the location of the emitter conduction band. As a result, different transparent front contacts were also investigated to determine a device structure required to produce a high performance CZT or CMT top-cell for tandems devices.